In optical fiber communication systems, optical signals propagating along an optical fiber undergo signal attenuation due to absorption and scattering in optical fibers. Therefore, optical signals require periodic amplification over long distances, which can be performed either by electrical repeaters or by optical amplifiers. Known optical amplifier types include Erbium-doped fiber amplifiers (EDFAs), semiconductor optical amplifiers and Raman amplifiers. Due to its low noise figure and its flat gain over a wide signal wavelength band, the Raman amplifier has gained increasing attention in the recent past as ideal amplifier candidate for use in wavelength division multiplex (WDM) signal transmission.
The Raman amplification process is based on the Raman effect, which is a non-linear optical process that occurs only at high optical intensities and involves coupling of light propagating through the non-linear medium to vibrational modes of the medium, and re-radiation of the light at a different wavelength. Re-radiated light upshifted in wavelength is commonly referred to as a Stokes line, whereas light downshifted in wavelength is referred to as an Anti-Stokes line. The Raman effect is described by quantum mechanics as scattering of photons at molecules which thereby undergo a transition of their vibrational state. Raman amplification involves stimulated Raman scattering, where the incident beam, having a higher optical frequency, often referred to as the pump beam, is used to amplify the lower frequency beam often referred to as the Stokes beam or the signal beam through the Raman effect.
In a silica fiber for example, the strongest Raman scattering, i.e. the maximum Raman gain occurs at a frequency shift of about 13.2 THz, which corresponds to a wavelength shift of about 50–100 nm for pump wavelengths between about 1 and 1.5 μm. A pumping scheme, where the pump beam is detuned from the signal beam by one Stokes shift is referred to as first-order pumping. First order pumping has, however, some limitations. The pump signal power launched into the fiber link will decrease exponentially with the distance as the light propagates into the fiber, which means that regardless of how powerfull the pump, most of the amplification will occur relatively near the point where the pump is injected into the fiber. This limits the improvement in the optical signal-to-noise ratio (OSNR) that the Raman amplifier can induce. Moreover, as the pump power is increased, Rayleigh scattering of the signal limits also the improvement in the OSNR.
A second-order pumping scheme has therefore been recently proposed, where a relatively strong second order pump beam, detuned by two Stokes shifts from the signal beam, is used to amplify a first order pumping beam which in turn serves to amplify the signal beam. U.S. Pat. No. 6,163,636 for example discloses an optical fiber communication system with one or more multiple-order Raman amplifiers. The amplifier injects a first-order pump beam and a second order pump beam into the transmission line. The article “Third-Order Cascaded Raman Amplification” by S. B. Papernyi et al, post-deadline digest OFC 2002, p. FB4, describes a third-order Raman amplifier with a pump that delivers a pump beam detuned by three Stokes shifts from the signal beam, which is injected in counter-propagating direction into the transmission line. The amplifier requires only a single active pump source while two Bragg fiber gratings are used to create the lower-order “seed” wavelengths from amplified spontaneous emission (ASE). Higher-order (i.e., second- or third-order) Raman amplifier systems have compared to first-order Raman amplifiers reduced noise, longer fiber span lengths, and reduced non-linearities.
Additionally, transmission systems require next to amplification also dispersion compensation since optical signals propagating along an optical fiber are subject to chromatic dispersion. Dispersion compensation is typically achieved by the use of dispersion compensating fibers (DCF), i.e., optical fibers with a negative refractive index at the wavelength spectrum of the signal beam. DFCs are in turn a source of signal attenuation and thus require signal amplification. It would equally be possible to use the DCF itself as a Raman gain medium but which would require another Raman pump for the DFC gain. A complete system would thus require a second-order Raman amplifier for amplification in the transmission line and a separate Raman amplifier for the DCF gain.
It is therefore an object of the present invention to provide a Raman amplifier with improved noise characteristic which is well suited for use in an optical fiber communication system with dispersion compensation.